Switching amplifiers enjoy significantly better efficiency than their non-switching predecessors, primarily because transistors used to switch voltages to a load are either turned ON, so that the voltage across the transistor is relatively low, or turned OFF, so that the current through the transistor is relatively low. With either a low voltage across the transistor or a low current through the transistor, the power dissipated by the transistor is relatively low.
Switching amplifiers using a single PWM stream are widely used. However, these switching amplifiers cannot accurately amplify high bandwidth, high accuracy signals at a reasonable cost. A more recent approach has been to use switching amplifiers capable of using modulation techniques that include two or more PWM streams, such as a multi-reference switching amplifier. A multi-reference switching amplifier is described in U.S. Pat. No. 6,535,058 Multi-reference, High Accuracy Switching Amplifier, the entire content being incorporated herein by reference. One example of a multi-reference switching amplifier comprising two separate PWM streams where one stream is a coarse high voltage PWM stream and the other stream is a fine low voltage PWM stream.
Switching amplifiers can sometimes generate excessive electromagnetic radio frequency (“RF”) interference that can interfere with the operation of the amplifier as well as with other electronic devices in the vicinity of the amplifier. This RF interference can be attenuated to some extent by coupling the load driven by the amplifier to low pass filters formed by inductors and/or capacitors. A low pass filter attenuates a signal beyond a specific frequency, often called the cutoff frequency. The greater the frequency of the RF interference is above the cutoff frequency, the more the RF interference is attenuated. However, decreasing the cutoff frequency to better attenuate the RF interference limits the bandwidth of the switching amplifier. Conversely, increasing the sampling rate to allow the filter to provide better attenuation of the RF interference reduces the available dynamic range of the output signal, such the ratio of the loudest signal to the smallest signal.
Therefore, there is a need to reduce RF interference without degrading the output signal and without compromising output signal dynamic range.